Chiral liquid crystal (LC) materials are useful for many applications, for example LC displays (LCD) or polymer films with a twisted structure. Usually they consist of an LC host material containing one or more chiral dopants which induce the desired helical twist. The effectiveness of a chiral compound to induce a helically twisted molecular structure in a liquid crystal host material is described by its so-called helical twisting power (HTP). The HTP is given in first approximation, which is sufficient for most practical applications, by equation (1):
                              H          ⁢                                          ⁢          T          ⁢                                          ⁢          P                =                  1                      p            ·            c                                              (        1        )            wherein c is the concentration of the chiral compound in the host material and p is the helical pitch.
As can be seen from equation (1), a short pitch can be achieved by using a high amount of the chiral compound or by using a chiral compound with a high absolute value of the HTP. Thus, in case chiral compounds with low HTP are used, high amounts are needed to induce a short pitch. This is disadvantageous, because the chiral compounds known from prior art do often negatively affect the properties of the LC host mixture like the clearing point, dielectric anisotropy, viscosity, driving voltage or switching times, and because chiral compounds can be used only as pure enantiomers and are therefore expensive and difficult to synthesize.
Another disadvantage of prior art chiral compounds is that they often show low solubility in the LC host material, which leads to undesired crystallization at low temperatures. To overcome this disadvantage, typically two or more different chiral dopants have to be added to the host mixture. This implies higher costs and does usually also require additional effort for temperature compensation of the material, as the different dopants have to be selected such that their temperature coefficients of the twist compensate each other.
Consequently, there is a considerable demand for chiral compounds with a high HTP which are easy to synthesize, can be used in low amounts, show low temperature dependence of the twisting power e.g. for utilizing a constant reflection wavelength, show good solubility in an LC host material and do not have a negative influence on the properties of the LC host.
The invention has the aim of providing chiral compounds having these properties, and not having the above-mentioned disadvantages of prior art chiral compounds. Another aim of the invention is to extend the pool of chiral compounds available to the expert. Other aims are immediately evident to the expert from the following description.
The inventors of the present invention have found that these aims can be achieved by providing chiral compounds as claimed in this invention, which comprise a 6,6′-bisalkinyl-1,1′-bi(2-naphthol) group.
Chiral binaphthol derivatives with alkinyl groups are disclosed in JP 2002-179668 A, JP 2002-179669 A, J. Am. Chem. Soc. 2001, 123(11), 2683, Chem. Phys. Letters 1996, 253(1,2), 141, Mol. Cryst. Liq. Cryst. S&T, Section B 1995, 9, 181, and J. Chem. Soc., Chem. Commun. 1994, 3, 249. However, compounds as claimed in the present invention are not disclosed.